In recent years, reduction in size and weight of electronic devices has been remarkably progressed, and with the progress, it has been demanded that secondary batteries that are used for such electronic devices should have higher energy density. An example of secondary batteries that can meet the demand is a secondary battery using a nonaqueous electrolyte solution (hereinafter, referred to as nonaqueous secondary battery) such as a lithium-ion secondary battery.
The lithium-ion secondary battery uses a nonaqueous electrolyte solution, and the nonaqueous electrolyte solution comprises an electrolyte salt such as a lithium salt and a nonaqueous solvent. The nonaqueous solvent is desired to have high dielectric constant and high oxidation potential, and to be stable in batteries regardless of operation environment.
As such a nonaqueous solvent, aprotic solvents are used, and known examples thereof include high-permittivity solvents such as cyclic carbonates including ethylene carbonate and propylene carbonate, and cyclic carboxylate esters including γ-butyrolactone; and low-viscosity solvents such as chain carbonates including diethyl carbonate and dimethyl carbonate, and ethers including dimethoxyethane. Usually, a high-permittivity solvent and a low-viscosity solvent are used in combination.
However, the lithium-ion secondary battery using a nonaqueous electrolyte solution may suffer from leakage of the nonaqueous electrolyte solution due to a defect involving increased internal pressure caused by breakage of the battery or any other reason. The leakage of the nonaqueous electrolyte solution may lead to short-circuit between a positive electrode and a negative electrode constituting the lithium-ion secondary battery to cause generation of fire or burning. It may also lead to generation of heat in the lithium-ion secondary battery to cause vaporization and/or decomposition of the organic solvent-based nonaqueous solvent to produce gas. In some cases, the produced gas caught fire or caused rupture of the lithium-ion secondary battery. In order to solve the above-described problems, studies have been carried out to give flame retardancy by adding a flame retardant to the nonaqueous electrolyte solution.
Techniques to add a flame retardant to a nonaqueous electrolyte solution is proposed in Japanese Unexamined Patent Application Publication No. HEI 6(1994)-13108 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2002-25615 (Patent Document 2), Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-525597 (Patent Document 3) and Japanese Unexamined Patent Application Publication No. HEI 11(1999)-329495 (Patent Document 4), for example.
As the flame retardant, specifically, Japanese Unexamined Patent Application Publication No. HEI 6(1994)-13108 and Japanese Unexamined Patent Application Publication No. 2002-25615 propose phosphazene derivatives, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2001-525597 proposes azobis(isobutyronitrile) (AIBN), and Japanese Unexamined Patent Application Publication No. HEI 11(1999)-329495 proposes imidazole compounds.